Fibers and ribbons for use in the manufacture of solar cells

ABSTRACT

This invention is directed to a process for the fabrication of features on a silicon wafer utilizing ribbons comprising organic polymer and inorganic material.

FIELD OF THE INVENTION

This invention is directed to a fiber or ribbon comprising organicpolymer and inorganic materials. The invention is further directed to aprocess for the fabrication of features on solar cell structuresutilizing such fibers or ribbons.

BACKGROUND OF THE INVENTION

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front side or sun side of the celland a positive electrode on the backside. The solar cell has acarrier-collecting junction close to its front surface. The frontsurface is contacted with parallel fingers which are currently about 140microns wide per finger. The fingers are connected by two bus bars thatare perpendicular to the fingers. Typically, a five-inch square cell hasabout 55 fingers separated by about 2.1 mm spaces. The electric currentcollected for storage by the solar cell is gathered by metal contacts toa doped region on the front surface and by a second contact to theoppositely doped region on the back surface.

It is difficult to obtain very fine line and space resolution for theformation of the negative electrodes when applied by conventionalpatterning techniques such as screen printing, sputtering or chemicaletching methods. The present invention will allow for the use of fibersor ribbons, wherein conductive metal particles are integrated into thefiber or ribbon, to form such electrodes on the front surface of thesolar cell structure. The fibers or ribbons will allow narrower lineswith increased height thickness which will increase the cell power bydecreasing cell shadowing loss without increasing resistance of thelines. Currently shadowing loss accounts for about 9% loss in a solarcell structure. Narrower lines will substantially decrease such loss.

U.S. Pat. Nos. 3,686,036, 4,082,568, 4,347,262, and 4,235,644 disclosevarious solar cell devices and methods of manufacture.

SUMMARY OF THE INVENTION

This invention provides a process for the manufacture of electrodes on asolar cell structure comprising the steps of affixing a fiber or ribboncomprising an organic polymer and a conductive material to a substratesuch as a silicon wafer in a desired orientation forming a solar cellstructure; heating the structure to a temperature above the meltingpoint of the organic polymer; and heating the structure to a temperatureto sufficiently effect the essentially complete removal of the organicpolymer resulting in the inorganic material affixed to the substrate inthe desired orientation.

The invention is also directed to a fiber or ribbon for use in themanufacture of electrodes on the front surface of a solar cell structurecomprising conductive particles, dielectric particles or mixturesthereof combined with a polymer suitable for forming a spinnabledispersion.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an illustration of a top surface metallization of a solarcell.

FIG. 2 is a cross sectional illustration of FIG. 1 with a press plate.

FIG. 3 is an illustration showing fiber placement after processing.

FIG. 4 is an illustration of the solar cell structure after firing.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to methods for the manufacture ofsolar cell structures utilizing fibers or ribbons bearing conductivemetal particles combined with a spinnable fiber-forming dispersion.

Examples of applications for the fibers or ribbons include themanufacture of features on a solar cell structure, for example, negativeelectrodes or emitters when the inorganic material of the fiber orribbon are conductive metal particles.

Polymers suitable for use in the present invention include those thatform a spinnable dispersion with particles of inorganic materials suchas conducting metal particles. The polymer must be soluble in a suitablesolvent so that a dispersion comprising polymer and inorganic materialparticle can be prepared. The resulting inorganic material bearingpolymer dispersion must be capable of being spun into flexible fibers orextruded into flexible ribbons. As used herein, the term “fiber” means asingle flexible filament; a group of flexible filaments twisted togetheror bundled together; or a group of flexible filaments lying parallel toeach other forming a bundle. The bundle may or may not be coated toafford protection to the fiber. Cross sectionally, the fiber or bundlemay be circular, oblong, square, rectangular, triangular and any othershape. The term “ribbon” means a flexible strip and may be onehomogeneous ribbon; or may be constructed by laying more than one fiberin the same plane; or may be several homogeneous ribbons or planes offibers or combinations thereof layered on top of one another to form aribbon-like structure. Although preferred that each fiber or ribbon havethe same chemical properties, there are some applications that may deeman intermingling of different chemistries of the individual fibers orribbons. Fiber diameters typically range from 20 microns to 100 microns,but may extend beyond the range for some applications. Ribbon sizetypically ranges up to 200 microns in width to 100 microns in height butmay extend beyond the range for some applications. Length of fiber orribbon is preferred to be continuous, but noncontinuous fiber or ribbonlengths for forming components is suitable. Further, the polymer must beselected so as to soften and melt cleanly; for example, the polymer mustburn out cleanly without leaving behind any residue. The polymer, in thepresence of the inorganic material, must exhibit a melting point priorto decomposition. The polymer melt that results from the chosen polymermust wet out the material on which the solar cell is constructed.Examples of polymers that meet the above selection procedure includeethylene/vinyl acetate copolymers, obtainable from the DuPont Company(Wilmington, Del.) under the trade name ELVAX®. Also,polymethylmethacrylate polymers may be suitable in the present inventionand are available from the DuPont Company (Wilmington, Del.) under thetrade name ELVACITE®. Additional suitable ethylene/vinyl acetatecopolymers and polymethylmethacrylate polymers are available frommanufacturers such as Dow and Exxon.

Organic solvents for use in the preparation of the spinnable inorganicmaterial/polymer dispersion are characterized by high solubility for thepolymer and by high vapor pressure at spinning temperatures tofacilitate spinning of the inorganic material into polymer fiber orribbon. Some examples of suitable solvents include tetrachloroethylene,toluene, and xylenes. Dry spinning is a preferred technique for forminga fiber or ribbon. However, wet spinning, melt or gel spinning can beemployed. All techniques are well known by those in the art of fibertechnology.

Wet spinning is the oldest process. It is used for fiber-formingsubstances that have been dissolved in a solvent. The spinnerets aresubmerged in a chemical bath and as the filaments emerge theyprecipitate from solution and solidify. Because the solution is extrudeddirectly into the precipitating liquid, this process for making fibersis called wet spinning.

Dry spinning is also used for fiber-forming substances in solution.However, instead of precipitating the polymer by dilution or chemicalreaction, solidification is achieved by evaporating the solvent in astream of air or inert gas. The filaments do not come in contact with aprecipitating liquid, eliminating the need for drying and easing solventrecovery.

In melt spinning, the fiber-forming substance is melted for extrusionthrough the spinneret and then directly solidified by cooling.

Gel spinning is a special process used to obtain high strength or otherspecial fiber properties. The polymer is not in a true liquid stateduring extrusion. Not completely separated, as they would be in a truesolution, the polymer chains are bound together at various points inliquid crystal form. This produces strong inter-chain forces in theresulting filaments that can significantly increase the tensile strengthof the fibers. In addition, the liquid crystals are aligned along thefiber axis by the shear forces during extrusion. The filaments emergewith an unusually high degree of orientation relative to each other,further enhancing strength. The process can also be described as dry-wetspinning, since the filaments first pass through air and then are cooledfurther in a liquid bath.

Conductive metal materials in powder form usable in the process of thepresent invention are those known by those in the electronics industry.Examples of suitable conducting metals, include those commonly used insolar devices, are as follows: Au, Ni, Au/Cr, Au/Cu, Au/Ta, Cu/Cr,Au/indium tin oxide, Cu, Ag and Ni. Ag is the preferred conductingmetal.

Fibers or ribbons comprising 70-80% conductive metal particles by weightare preferred; although, inorganic particle loading up to 90% by weightis suitable. Maximizing the inorganic solids loading will minimizeshrinkage of the fibers during firing. However the polymer content mustbe kept high enough to form a flexible and uniform fiber or ribbon.

The method of affixing the formed fiber or ribbon to the solar cellsurface (substrate) may be any method ranging from manual placement tomechanized placement means. The solar cell surface may be any surfacesuitable for use in solar cell manufacturing, but the preferred surfacesare processed Si wafers, CuInSe substrates, or substrates covered withthin films of amorphous silicon, CD-In—Ga—Se, CdS, ZnS, or CdTe.

After placement of the fiber or ribbon, the first heating stage heatsthe solar cell structure that holds the affixed fiber or ribbon to atemperature above the melting point of the organic polymer component ofthe fiber or ribbon. This adheres the inorganic material/polymer fiberor ribbon to the substrate material. In one embodiment the heatingsource may be incorporated into a press plate. In another embodiment theheating source may be independent of the press plate.

A second heating stage which substantially to completely burns out theorganic polymer from the fiber or ribbon results in the inorganicmaterial adhering to the substrate material. It is preferred that theconductive inorganic materials are sintered. The firing temperatureprofile may be the two heating stages carried out in one continuum ofheating or two discretely staged heating events. For example, the fiberand/or ribbon placement and initial heating stage can be combined.Heating either the substrate and/or fiber/ribbon during the placementstep can be used to secure the fiber/ribbon in the required position onthe substrate. This would be the preferred positioning method to createfunctional features where there is no mechanical support structure tohold the fiber and/or ribbon in place. Another alternative method totack fibers and/or ribbons in place without heating would be to wet thefiber or substrate surface with solvent vapors immediately beforepositioning (to make the fiber stick to the substrate).

More specifically, FIG. 1 illustrates a top surface metallization by theplacement of multiple fine fibers (101) of conductive material onto asolar cell.

The fibers are very narrow having dimensions of about 20 to 80 micronsin width. The fibers can be extruded as either circular rods orrectangular shaped ribbon. Their aspect ratio may typically be about 1.

The fibers may be placed by a fixture such as a loom (102) andpositioned over the solar cell substrate such as a processed crystallineSi wafer (103). Proper positioning will need to be done accurately (X-Ypositioning shown in FIG. 1). A cross-sectional view of FIG. 1 is shownin FIG. 2. FIG. 2 positions the fibers close to the Si wafer surfaceprior to pressing with a heated TEFLON® plate (201).

Fiber number and spacing will vary as a function of solar cell design.Some designs may have close spaces between lines as illustrated inFIG. 1. Some designs may have wider spaces between lines.

The placement of fibers is a critical operation. The narrow fibers willneed support, as they may be fragile. They will need to be positionedaccurately with repeated performance.

Once the fibers are placed onto or above the cell, a press plate mayboth press and heat the fibers to stick them to the cell surface (FIG.2). The TEFLON® plate will not grip the fibers thereby allowing theplate to be removed without pulling on the fibers. Prior to lifting theplate the fibers will typically be cut. In another embodiment, anindependent heating source may be used from the underside of the cell.

The cell has a thin film antireflective coating of SiNx (202) on itsupper surface. Under the SiNx is a diffused layer of n+Si(203)(typically Si with high concentration of P n-type dopant [for anegative emitter]). The base of the cell is p-type Si (103).

FIG. 3 shows the cell cross section of FIG. 2 after the press plate isremoved. The fibers (101) have been deformed slightly during pressingand heating and have been made to adhere to the top of the SiNx layer(202) (the outermost surface of the cell).

FIG. 4 illustrates the cell cross section of FIG. 3 but after firing thefibers (101). The fibers contact the n+Si layer (203) by penetrating theSiNx layer (202). This is necessary, as the SiNx is a non-conductinglayer. FIG. 4 also illustrates the fibers remaining in the same X-Yposition after firing.

EXAMPLES Example 1

A smooth fiber forming paste containing silver particles was made asfollows. 5.0 g ELVAX® 265 ethylene vinyl acetate resin (DuPont,Wilmington, Del.) was first soaked with 30 ml tetrachloroethylene (TCE)in a 100 ml beaker for one half hour. The beaker wrapped with a roundband heater was enclosed in a bell jar. An air-driven stirrer wassituated at the center of the bell jar for stirring the mixture in thebeaker. The mixture was heated to 100° C. until the polymer dissolved.To the solution, 15.0 g silver powder (silver powder, nominal size ˜2 μmD50 [˜0.5 μm D10, ˜7 μm D90] available from E. I. du Pont de Nemours andCompany, Wilmington, Del.) and 0.4 g glass frit were added. It wasstirred for about four hours. Once the mixture looked very smooth,vacuum was applied to the bell jar to thicken the mixture until anextensible viscosity was obtained. The mixture was then tested with aspatula to ascertain that fiber could be pulled from the smooth, thickpaste. Once a smooth, fluid mixture was achieved, it was transferred toa plastic syringe having a ˜0.5 mm diameter hypodermic needle forextrusion. The paste had to be kept at ˜80° C. while it was beingextruded to TEFLON® fluoropolymer sheets (DuPont, Wilmington, Del.) forforming continuous fibers. Fibers ranging from 100 to 300 microns wereobtained. The obtained fibers were elastic and could be handled easilywithout breaking, which makes it possible for further processing. Thinribbons down to 50 microns in thickness were obtained if the extrudatescame into contact with the TEFLON® fluoropolymer sheets (DuPont,Wilmington, Del.) before the skin of the extrudate had solidified.

Glass Frit Composition (given in weight %)

Bi₂O₃ 82.0

PbO 11.0

B₂O₃ 3.5

SiO₂ 3.5

Milled to nominal S.A. of 5.5 m²/g

Example 2

The silver fibers from Example 1 were affixed to a silicon wafer andfired at 900° C. (set point) in an IR furnace. The fibers did not retaintheir original dimensions. The fired fibers measured 40 microns×8microns high.

1. A ribbon for use in the manufacture of features for a solar cellstructure comprising conductive inorganic material combined with apolymer suitable for forming a spinnable dispersion.